Electronically tuneable fiber-optic receiver for narrow band microwave signal reception

Abstract

The optical receiver has a photodiode (10) which is reverse biased by a voltage supply (14). The voltage supply provides a variable bias voltage determined by a control unit (16) and the photodiode is matched to the load (22) by an impedance matching circuit (12). The photodiode exhibits large capacitance changes over a range of bias voltages and may be implemented using a Schottky barrier or P + N photodiode. By changing the bias voltage, the photodiode capacitance changes to vary the tuned frequency of the receiver. The matching circuit cancels the reactive component of the photodiode impedance and matches the resistive component to the load. The photodiode may have a doping profile in which an intrinsic or lightly doped region of width greater than the average photon penetration depth is located next to the junction. After the intrinsic region, the doping profile may be selected to achieve linear tuning. This doping profile gives linear tuning without sacrificing photodiode conversion efficiency.

What is claimed is: 
     
       1. An impedance matched narrow-band optical signal receiver comprising: a photodiode having a depletion capacitance which varies as a function of the amount of bias voltage applied thereto, an electronic impedance matching circuit disposed between said photodiode and a load, said electronic impedance matching circuit matching said photodiode to said load over a narrow bandwidth having a center microwave frequency;   a variable power supply, said variable power supply being connected to said photodiode and providing said bias voltage thereto;   means for shifting said center microwave frequency of said narrow band receiver over a band of microwave frequencies, said shifting means being connected to said variable power supply and controlling the output thereof to vary the depletion capacitance of the photodiode until it resonates with the impedance matching circuit at the microwave frequency to which the receiver is desired to be tuned.   
     
     
       2. The modulated optical signal receiver as claimed in claim 1 wherein said electronic impedance matching circuit is a two-element circuit having an inductor serially connected with a resistor. 
     
     
       3. The modulated optical signal receiver as claimed in claim 1 wherein said electronic impedance matching circuit includes a matching inductor for substantially cancelling the reactance of said photodiode and a quarter-wavelength transformer for matching the resistance of said photodiode to said load. 
     
     
       4. The modulated optical signal receiver as claimed in claim 1 which further comprises: first means for isolating said variable power supply and said center microwave frequency shifting means from microwave signals, said first isolating means being disposed between said variable power supply and said electronic impedance matching circuit.   
     
     
       5. The modulated optical signal receiver as claimed in claim 4 further comprises: second means for isolating said load from said bias voltage, said second isolating means being disposed between said electronic impedance matching circuit and said load.   
     
     
       6. The modulated optical signal receiver as claimed in claim 1 wherein said photodiode is a P +  N, N +  P or Schottky barrier semiconductor photodiode. 
     
     
       7. The modulated optical signal receiver as claimed in claim 1 wherein said photodiode has a doping profile to produce a linear relationship between variations of said bias voltage and the shifting of said center microwave frequency. 
     
     
       8. The modulated optical signal receiver as claimed in claim 1 wherein said photodiode includes a junction and a first region adjacent said junction, said first region being of a width greater than the average photon penetration depth. 
     
     
       9. The modulated optical signal receiver as claimed in claim 8 wherein said first region is an intrinsic semiconductor material. 
     
     
       10. The modulated optical signal receiver as claimed in claim 8 wherein said first region is a lightly doped semiconductor material. 
     
     
       11. The modulated optical signal receiver as claimed in claim 8 wherein said photodiode has an intrinsic region and has a doping profile N in accordance with the following equation:   N=N.sub.o X.sup.-3/1, for X>0     where N o  is the peak doping concentration at x=0 and X is the distance from the end of said intrinsic region.